The need to transmit ever increasing amounts of data necessitates the development of optical communication systems that combine dense wavelength division multiplexing (DWDM) of optical channels with advanced spectral-efficient modulation formats incorporating phase modulation. One of the data transmission technologies considered is the Dual-Polarization Quadrature Phase-Shift Keying, or “DP-QPSK”, in combination with the coherent detection at the receiver. The DP-QPSK modulation format includes transmitting two orthogonally polarized light signals, each phase-modulated with two quadrature ‘I’ and ‘Q’ signals according to the QPSK modulation format. The advantages of this technique, in addition to the spectral efficiency, includes robustness with respect to the chromatic dispersion (CD) and the polarization mode dispersion (PMD) in optical fibers.
Besides having intrinsically large PMD and CD tolerance, the coherent detection of the phase-modulated light at the receiver can also be used to implement “colorless receivers” and therefore “colorless networks”, that is networks without specific wavelengths assigned to individual add/drop (A/D) ports; see for example an article “Combining 40G DP-QPSK with 10G OOK channels on metro/regional networks” by N. L. Swenson, Lightwave®, December 2012. With a coherent detection, the receiver may be able to select a desired channel from multiple received channels simply by tuning a local oscillator (LO) laser without the need for an optical DEMUX, a wavelength selective switch (WSS), or a reconfigurable add-drop module (ROADM), thereby reducing optical loss in the network, greatly simplifying the network, an making it potentially less expensive. This can reduce the number of ROADM/WSS elements required in the network and dramatically simplify network topology. However, wavelength tuning in most commercially available tunable lasers that can be used as local oscillators in a tunable coherent receiver is relatively slow, so that tuning or switching the LO laser wavelength from one DWDM channel to another takes tens of seconds or even minutes. Furthermore, the slow channel switching times make the receiver auto-configuration procedure too long for many real-life network applications, as it may involve scanning the entire channel spectrum to identify all the received channels.
An object of the present invention is to overcome the shortcomings of the prior art by providing a method for fast channel detection and/or auto-configuration at a coherent receiver, and a tunable optical receiver implementing the method.